HISTAMINE ENHANCES VAGAL PULMONARY C-FIBER RESPONSES TO CAPSAICIN AND LUNG-INFLATION

Effects of histamine on the base-line activity of vagal pulmonary C-fibers and their responses to chemical and mechanical stimulations were studied in anesthetized, open-chest dogs. Histamine aerosols (5 breaths, 1% solution) increased the change in tracheal pressure per breath (DELTAP(t)) to 53 +/- 9% (mean +/- SE) above its base line, whereas an accompanying increase in activity was found in only 27% of the C-fibers studied and hence the overall afferent activity was not significantly different from the control. After P(t) returned toward the base line in several minutes, the receptor activity evoked by right atrial injection of capsaicin (0.8-4.0 mug/kg) increased from a base line of 0.43 +/- 0.02 imp/sec to 8.12 +/- 1.16 imp/sec (averaged over 10 sec), which was markedly greater and longer lasting than the response triggered by the same dose of capsaicin after phosphate buffer aerosols (4.72 +/- 0.71 imp/sec). Furthermore, pretreatment with histamine aerosols also enhanced the afferent responses of these receptors to static lung inflation (P(t) = 20 cmH2O). We conclude that the low dose of histamine aerosols alone did not consistently stimulate the vagal pulmonary C-fibers, but it potentiated the stimulatory effects of both capsaicin and lung inflation on these receptors.